A solution with a pH value less than 7 is a/an:

Correct — C, Acidic solution. The pH scale runs from 0 to 14 and measures the concentration of hydrogen ions, which in water exist as hydronium ions. Seven is the middle, and it is where a neutral solution sits because pure water at ordinary temperature contains equal and very small concentrations of hydronium and hydroxide ions. Anything that raises the hydronium concentration pushes the number down, so a reading below 7 means an acid, and the further below, the stronger the acid — the scale is logarithmic, so pH 3 is ten times as acidic as pH 4 and a hundred times as acidic as pH 5. Above 7 the hydroxide concentration is rising and the solution is basic. Everyday anchors make the scale concrete: gastric juice in the stomach is around 1 to 2, lemon juice about 2.5, milk close to 6.5, blood a little above 7 and tightly held there, milk of magnesia about 10, and a sodium hydroxide solution up at the top.

• (a)Neutral solution — Neutral is the value 7 itself, not anything below it — the point at which hydronium and hydroxide ion concentrations are equal, as in pure water.
• (b)Basic solution — The other half of the scale, above 7. A rising pH means a rising hydroxide ion concentration, which is the opposite of what a reading below 7 shows.
• (d)Hypotonic solution — Belongs to a different measurement entirely. Hypotonic describes a solution whose solute concentration is lower than that inside a cell, so water moves into the cell — a matter of osmosis, not of hydrogen ions, and a solution can be hypotonic at any pH.

The letter p in pH stands for the German potenz, meaning power, and the scale is a way of writing hydrogen ion concentration as a small number. Higher the hydronium ion concentration, lower the pH. Universal indicator paper gives a colour that maps onto the scale; laboratory precision comes from a pH meter. Strong acids and strong bases ionise almost completely in water and so give extreme readings, while weak acids and weak bases ionise only partly and cluster nearer the middle.

This is a definitional item, and the useful work lies in what surrounds it rather than in the answer. Two habits pay off in the exam hall. First, hold a handful of anchor values so that any pH question can be placed by comparison rather than by recall — stomach acid near 1, lemon near 2.5, pure water at 7, blood just above it, milk of magnesia near 10. Second, remember that the scale is logarithmic, because a large share of pH questions turn on that single fact rather than on the definition. The odd option out here, hypotonic, is a reminder that the paper will sometimes drop a term from an entirely different chapter into the list; recognising which measurement a word belongs to is often enough to eliminate it.

• The pH of a neutral solution is 7; values below 7 are acidic and values from 7 to 14 are increasingly basic.
• Higher the hydronium ion concentration, lower the pH value — the scale is inverse and logarithmic, so one pH unit is a tenfold change.
• Human blood is held in a narrow band a little above 7, and living systems are sensitive to small shifts.
• Tooth enamel begins to corrode when the pH in the mouth falls below about 5.5, which is why basic toothpastes help.
• Rain becomes acid rain when its pH falls below about 5.6, and salts of a strong acid and weak base give acidic solutions with pH below 7.

• Treating the scale as linear; a drop of two pH units is a hundredfold rise in acidity.
• Assuming a strong acid always has a lower pH than a weak one regardless of concentration — strength and concentration are different things.
• Mixing up pH with tonicity or with concentration in general, which is exactly what the fourth option invites.

As this direct definition, as an anchor-value question about a named substance, or through an application such as tooth decay, acid rain or the treatment of acidic soil.

• practice — not a real PYQ

  If the pH of a solution changes from 5 to 3, the hydrogen ion concentration becomes

  • (a)2 times greater
  • (b)10 times greater
  • (c)100 times greater
  • (d)100 times smaller

  Answer(c) 100 times greater — the pH scale is logarithmic, so each unit fall multiplies the hydrogen ion concentration by ten, and a fall of two units multiplies it by a hundred.

• practice — not a real PYQ

  Which one of the following is used to treat soil that has become too acidic for a crop?

  • (a)Dilute hydrochloric acid
  • (b)Quicklime or slaked lime
  • (c)Common salt
  • (d)Ammonium sulphate

  Answer(b) Quicklime or slaked lime — these are basic, so they neutralise the excess acid and raise the soil pH towards the range the crop needs.